ISTANBUL, March 10 (Reuters) - A spate of attacks on Turkey's pro-Kurdish People's Democracy Party (HDP) is harming its ability to campaign for local elections later this month and threatens the fairness of the outcome, party leaders said on Monday.

Public officials, including members of the police, are involved in the attacks in an attempt to stoke Turkish nationalist sentiment ahead of the March 30 mayoral elections, Ertugrul Kurkcu, HDP co-chairman, also told a news conference.

On Sunday, dozens of men threw rocks through the windows of an HDP office in the Mediterranean city of Fethiye and tried to tear down a party emblem from an outside wall, footage from the IMC television channel showed.

Police formed a barricade to hold back an angry crowd waving Turkish flags and numbering in the hundreds, although they did not prevent a few dozen approaching the building with sticks and stones. In the end, firefighters using an engine ladder tore off the HDP sign and threw it onto the ground.

Kurkcu said the incident in Fethiye was the 20th attack on their party since the start of the year.

"The security of this election has been destroyed," Kurkcu said. "These attacks are not happening on their own. Behind each attack has been a public security official."

The HDP is a sister party of the Peace and Democracy Party (BDP), which dominates much of the mainly Kurdish southeast. The HDP was formed late last year to fight the mayoral election in western Turkey, where Kurds are in a minority.

The BDP and HDP support peace talks between the state and Abdullah Ocalan, the jailed leader of the Kurdistan Workers Party (PKK) to end a three-decade Kurdish insurgency.

"UNACCEPTABLE ATTACKS"

Negotiations with Ocalan, who is serving a life sentence on an island off the Istanbul coast, are a deeply contentious issue for many Turks who blame the militant leader for the deaths of 40,000 people during the conflict.

Ocalan called on Prime Minister Tayyip Erdogan's government to stamp out the forces driving the attacks on the HDP and said they were aimed at disrupting the peace process, according to a statement released by an HDP delegation that met with him under the framework of the peace talks.

"All democratic elements and our people should take the necessary measures in the face of these unacceptable attacks that are testing the (peace) process," Ocalan said.

The PKK took up arms against the Turkish state in 1984 in a bid to carve out an ethnic homeland, but has since scaled back its demands to greater political autonomy and cultural rights.

The PKK called a ceasefire in March 2013 to support the peace talks, seen as the best chance yet to end the violence.

Kurkcu said the HDP will apply to the Organisation for Security and Organisation in Europe (OSCE), a rights watchdog, to help ensure security during the election and to ask the European Parliament to provide observers during the balloting.

HDP officials have received death threats, have had their buses pelted with rocks and offices destroyed by looters, according to a party statement. (Editing by Gareth Jones)

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